The science of the WG 2
- The history of black holes science largely identifies with the history of one of its subclasses
___ the science of stellar black holes.
The idea of quantum black holes could not flourish until theoretical physics was advanced enough to successfully combine quantum theories with general relativity ___ this happened only quite recently. Supermassive black holes were discovered by astronomers before any theorist could hypothesize their existence. Probably, if the observational evidence didn't show otherwise, nobody would have ever imagined that such huge concentrations of mass could form ___ even today, the mechanisms which bring to their formation are unknown. Stellar black holes, instead, appeared as a theoretically plausible outcome of massive stars evolution, soon after Einstein demonstrated that mass warps spacetime. On a scientific basis, this conclusion was almost straightforward. The volume of a star is constant as long as the thermal pressure balances the gravitational attraction. When the nuclear combustion ends, the temperature of a star drops, and so does its thermal pressure. The star is forced to contract. In 1930, it was already known that a quantum effect called electron degeneracy could stop the contraction and stabilize the star into a white dwarf. Chandrasekhar demonstrated that this was only possible for stars whose mass is smaller than ~ 1.4M☉. For more massive stars ___ he hypothesized ___ no force could stop the gravitational implosion. They would necessary evolve into a black hole.
Chandrasekhar's conclusion was not completely correct. At the time, he could not know that another quantum effect, the neutron degeneracy, would balance the gravitational attraction even for stars more massive than 1.4M☉, allowing them to evolve into another stable object ___ a neutron star. It is still unclear what exactly is the maximum mass (known as the Tolman-Oppenheimer-Volkoff limit) for which the neutron degeneracy is able to balance the gravitational force. Likely, it is not much higher than 2M☉. There is a considerable fraction of stars that, at the last stage of their life, will have masses above the Tolman-Oppenheimer-Volkoff limit. For them, the only possible evolution is to become a black hole.
Observing stellar black holes is not a trivial task. It took time, imagination and the evolution of X-ray telescopes to develop a method to identify them. A stellar black hole can be recognized when it interacts with another object; particles which flow continuously from the so called donor (usually a star), accelerated to relativistic speeds by the gravitational force of the black hole (the accretor), collide at very high energy, radiating X-ray photons. This process is not peculiar to black holes; white dwarfs and neutron stars can interact with companion objects in a similar fashion. The system of an accretor and a donor is called an X-ray binary. Once an X-ray binary is detected, it is possible to establish whether it contains a black hole by calculating the masses of the components.
Unlike supermassive black holes, binary star systems comprising either a stellar black hole or a neutron star are relatively common in the MilkyWay ___ this argument alone would suffice to explain their importance for testing general relativity. But they are also believed to be strong sources of gravitational waves ___ ripples in the curvature of spacetime which propagate through space. Among the Einstein's predictions that still need to be directly verified, gravitational waves are probably the most significant, and X-ray binaries may be essential to reveal them.
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